Optical coatings make use of the principles of optical interference, which deal with modifications in the transmitted and reflected intensities of light that occur when two or more beams of light are superimposed. For example, the brilliant colors that are seen when light is reflected from a thin layer of oil floating on water are produced by interference effects between the light waves reflected at opposite surfaces of the thin film of oil.
One important practical application of thin films involves the production of coated optical surfaces. If a film of a transparent substance having an appropriate thickness and refractive index is deposited on a lens, for example, the reflection of particular wavelengths of light from the lens surface can be almost completely suppressed. The light that otherwise would be reflected is not absorbed by such an antireflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a corresponding increase in the intensity of the light that is transmitted. The beneficial effects of thin film coatings, such as antireflection, are so desirable that substantially all high quality optical components are provided with optical coatings.
As optical coating technology has developed, improvements have been achieved through the introduction of multiple layer films. Two different materials are typically used in fabricating multiple layer films, one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited in a controlled sequence of thicknesses to obtain the desired optical characteristics for the film. The deposition process is typically controlled by monitoring the thickness of each layer as it is deposited and by terminating the deposition when the layer reaches the correct thickness. This approach provides the flexibility to design a wide range of multiple layer interference coatings for various transmission and reflection spectra. As a result, complex spectral filter structures have been added to many new optical devices. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band-rejection filters are some of the examples of useful devices employing multilayer thin film interference coatings.
Some advanced applications of optical technology, however, have performance requirements that exceed the capabilities of multiple layer thin films. New optical design procedures have been developed for these advanced applications to predict the continuous refractive index profile required for any desired transmission or reflection spectrum. These design techniques employ gradient index layers, in which the index of refraction varies continuously as a function of depth into the layer. Gradient index optical coatings have advantages over conventional technologies, including flexibility in filter design and increased stability in adverse environments. For example, the absence of discrete interfaces is predicted to lead to greater resistance to laser damage.
One type of gradient index structure is the rugate filter, the simplest manifestation of which has a periodic refractive index that varies sinusoidally with respect to optical thickness. A rugate filter is a gradient index analog of a quarterwave stack reflector. Compared to a quarterwave stack, a rugate filter has greatly suppressed high-frequency reflection harmonics. The rugate structure provides high reflectivity within a narrow bandwidth simply by increasing the number of periods in the filter.
Practical realizations of the rugate and other gradient index structures have been inhibited by the limitations of thin film fabrication technology. These limitations make it difficult to ensure that a fabricated coating accurately implements the theoretically specified refractive index profile. One prior method described in U.S. Pat. No. 4,707,611, which is incorporated herein by reference, measures the reflectance of two different wavelengths of light to determine the thickness and refractive index of an incremental thin film layer deposited on a base stack of layers. However, when a coating specification calls for a continuous refractive index profile, the thickness monitoring techniques of the prior art do not provide sufficient accuracy to ensure that the deposited layers will conform reliably to the specified profile. A slight error in the deposition thickness of a portion of a rugate filter, for example, can introduce a phase shift that may have a significant detrimental effect on the filter spectral structure. Also, an error in the refractive index of such a filter will add additional frequency components to the spectral profile, resulting in the generation of unwanted sidebands in the transmittance or reflectance spectrum. It is very difficult to compensate for such perturbations by any changes in the deposition of the remaining portion of the filter. Consequently, a need has developed in the art for an improved method of monitoring and controlling the deposition of optical thin films having continuous refractive index profiles.